1. Field of the Invention
The present invention relates to a voice coil motor device for positioning, and more especially to a voice coil motor device for positioning used for driving lenses for micro image capturing modules.
2. Description of Related Art
Recently, more and more handheld devices have image capturing modules. With the increasing requirements of the handheld devices for better performance and smaller size, the image capturing modules are required to have high image quality and small size. To improve image quality heeds to increase pixels of images on one hand. The pixels of image capturing modules for the handheld devices are gradually increased to one million pixels, two million pixels, three million pixels, even four million pixels and five million pixels from 300 thousand of VGA levels. On the other hand, definition of captured images must be improved. So the image capturing modules of the handheld devices are developed from fixed focus to optical auto-focus like cameras, even optical zoom.
The operation principle of optical auto-focus is to moving a lens of an image capturing module properly according to a distance from an object so that an optical image of the object can be accurately focused on an image sensor to produce a clear image. Conventional methods for driving the lens of the image capturing module include a stepping motor driving method, a piezoelectric driving method, a voice coil motor (VCM) driving method and so on.
Generally, voice coil motor mechanisms are mainly formed by placing coils in magnetic circuits with permanent magnets. According to Fleming's left-hand rule, when the coils are electrified, they interact with the permanent magnets to produce electromagnetic thrusts to move supporting bases connected with the permanent magnets, thereby driving lens elements fixed on the supporting bases. Optical auto-focus and optical zoom can be achieved via adjusting electric currents flowing the coils.
However, besides using voice coil motors to drive lens elements to move axially, conventional image capturing modules further need positioning mechanisms for effectively positioning the lens elements to ensure that optical images of objects are accurately focused on image sensors of the image capturing modules to produce clear images.
Conventional positioning methods mainly determine the positions for moving lenses depending on judging if the light is occluded. Though the methods can effectively achieve the positioning, they need to use a plurality of elements to cooperate with each other. So the image capturing modules have too large size, which limits the development of miniaturizing application products. Additionally, the detection results of the light occlusion positioning methods aren't voltage signals, converters are needed to convert the detection results into voltage signals, which increases costs.